'280 THE ICE AGE AND ITS WORK. 



Oil the whole, considering tlieii abundance in all glaciated regions 

 and the amount onntbnuation they give as to the direction and grind- 

 ing power of ice, these rounded rocks afford one of the most instructive 

 indications of the former presence of glaciers; and we must also agree 

 with the couchisiou of Darwin (in a paper written after studying the 

 phenomena of ice-action in North Wales, and while fresh from his obser- 

 vation of glaciers and icebergs in the Southern Hemisphere) that " one 

 of the best criterious between the effects produced by the passage of 

 glaciers and of icebergs is boss or dome-shaped rocks." 



(3) Striated, grooved, and tinted rocks, though ciosely connected 

 with the preceding, form a distinct kind of evidence of the greatest 

 value. Most of the bosses of rock just dcsciibed have been exposed to 

 the action of the atmos[>here, perhaps since the ice left them, and have 

 thus become more or less roughened or even disintegrated; but where 

 the rocks have been protected by a covering of drift, or even of turf, 

 and have been recently exposed, they often exhibit numerous i)arallel 

 stri;c, varying from the finest scratches to deep furrows a foot or more in 

 diameter. - - Perhaps none of the effects of ice so clearly demon- 

 strate the action of glaciers as opposed to that of icebergs, owing to 

 the general constancy of the direction of the stii.e, and the long dis- 

 tances they may be traced up and down slopes, with a steadiness of 

 motion and evenness of cutting power which no tloating mass could 

 l)ossibly exert. - - - 



(4) Erratic blocks were among the phenomena that first attracted the 

 attention of men of science. Large masses of granite and luird ineta- 

 morphic rock, which can be traced to Scandinavia, are found scattered 

 over the plains of Denmark, Prussia, and northern Germany, where 

 they rest either on drift or on quite different formations ol' the Second- 

 ary or Tertiary periods. One of these blocks, estimated at 1,500 tons' 

 Aveight, lay in a marshy plain near St. Petersburg, and a portion of it 

 was used for the pedestal of the statue of Peter the Great. In parts 

 of north Germany they are so abundant as to hide the surface of the 

 ground, being piled up in irregular masses forming hills of granite 

 bowlders, M'hich are often covered with forests of pine, l>irch, and 

 juniper. Far south, at Fiirstenwalde southeast of Berlin, there was 

 a huge l)lock of Swedish red granite, from one-half of which the gigan- 

 tic basinwas wrought which stands before the New ^Museum in that 

 city. - - - 



It is however in Switzerland that we find erratic blocks which fur- 

 nish us with the most conclusive testimony to the former enormous 

 extensicm of glaciers; and as these have been examined with the great- 

 est care, and the facts, as well as the main inductions from the facts, 

 are generally admitted by all modern writers, it will be well to consider 

 them somewhat in detail. It will be found that they give us most 

 valuable information both as to the depth and extension of ancient 

 glaciers, and also as to tlie possiltilities of motion in extensive ice- 

 sheets. 



